Thermal desorption methods to remove volatile contaminants from soils in situ are suggested in, for example, U.S. Pat. Nos. 4,973,811, 5,076,727, 5,152,341, 5,190,405, 5,193,934, 5,221,827, and 5,271,693. Methods of applying heat include microwave and radio frequency electrical power along with resistance heating between electrodes; injection of hot gases; and conduction of electricity through the soil. Conductive heat transfer from heat injection wells are suggested in, for example, U.S. Pat. Nos. 5,190,405 and 5,271,693. U.S. Pat. No. 5,271,693 suggests a heat injection well through which vapors are extracted from the formation.
These methods generally rely on maintenance of a low pressure at the surface or at a vapor extraction wellbore to control movement of contaminants from their initial position to a point where they can be recovered. When the contaminants are substantially immiscible with water and are more dense than water, the contaminants tend to sink downward and collect on clay or other impermeable layers. These contaminants are referred to as DNAPLs (dense nonaqueous phase liquids). These layers of clay often lay along old river beds that have been covered by sediment. These clay surfaces are generally covered with depressions (like a muffin tin) which act as traps for the DNAPL. Pools of DNAPL therefore are often thin and scattered along low spots on the layer of clay.
The most common remediation method for such contaminates is to put a well where it is thought that depressions lie in the clay surface, and to pump liquids out of the well until no more contaminants are produced. Old river bed channels or other depressions could therefore be inches away from the point the wellbore is placed, and the contaminants laying in the channel could be unrecoverable. DNAPLs generally contaminate ground water by slowly dissolving in the ground water. Attempts to remove DNAPLs by pumping and treating ground water are very slow because of limited solubility of DNAPLs in water. Such sites therefore must be maintained for extended periods of time. Further, it is not always convenient or possible to put wellbores above each suspected low spot.
It is therefore an object of the present invention to provide a method to remove DNAPLs from pools on impermeable layers. It is a further object to provide such a method wherein the contaminates can be removed in a relatively short period of time, and wherein numerous wellbores above pools of DNAPLs are not required.